1. Field of the Invention
The present invention generally relates to automotive and other vehicles, and more specifically to a wheel for such a vehicle which includes a pump for automatically inflating a tire mounted on the wheel.
2. Description of the Related Art
Low tire pressure is a major cause of excessive fuel consumption, tire wear and impaired steerability. A normal tire will typically leak on the order of 25 percent of its pressure per year due to its inherent permeability. It is thus good practice to maintain tire pressure on a regular basis.
However, even checking tire pressure every few weeks may not prevent these adverse affects when a slow leak is present, and the leak may go undetected unless a careful record is maintained of how frequently the pressure in each tire has to be replenished. A fast leak or flat condition can rapidly cause damage to the tire and even render it unusable in a short period of time, but this condition may go unnoticed by an inexperienced driver until it is too late.
It is thus highly desirable to have some mechanism that automatically replenishes the tire pressure when it is too low. Prior art tire pumps which are mounted on vehicle wheels and utilize centrifugal force to automatically pump air from the atmosphere into a tire and thereby maintain the tire pressure at a predetermined value are known in the art per se, such as disclosed in U.S. Pat. No. 1,254,903, entitled "AUTOMATIC TIRE PUMP" issued Jan. 29, 1918 to C Hall, and U.S. Pat. No. 4,349,064, entitled "INERTIAL TIRE PRESSURE REGULATORS" issued Sep. 14, 1982 to G. Booth.
The Hall and Booth devices are two-stage pumps which each include a piston which is radially movable in a cylinder to draw air from the atmosphere into a primary chamber and pump air from a secondary chamber into the tire when the piston is moved outward by centrifugal force resulting from movement of the vehicle and rotation of its wheels. The piston is moved inward by a spring when the vehicle stops to transfer air from the primary chamber into the secondary chamber.
In order to keep the mass of the piston and the force and size of the spring within practical limits, the piston and spring must be made small enough that the piston will begin to move outward in response to a small centrifugal force resulting from a low vehicle speed.
This causes a problem when the vehicle is operated at low speed in the rain, and/or on terrain including loose particulate matter such as dirt or sand. If the pump does not have an inlet filter, operation under such adverse conditions can cause these contaminants to be drawn into the pump to clog the inlet and outlet valves and even be pumped into the tire. If the pump does have an inlet filter, the filter can become clogged. These conditions can render the pump inoperative.
Friction between the piston and the wall of the cylinder when the pump is operating causes wear and reduction of the service life of the pump. Since tires leak slowly under normal conditions, an automatic tire pump is only required to operate during a fraction of the time the vehicle is running to maintain the tire pressure at the normal value. However, prior art tire pumps operate continuously, and are thereby subjected to more wear than is necessary.